It is to be understood that in the present document (unless the context requires otherwise) references to porous “low dielectric constant” (or “low-k”) structures denote structures in porous materials having a dielectric constant lower than about 3.0. Thus, these references encompass structures etched in porous so-called “ultra-low-k” (ULK) materials (having k≦2.5).
The fabrication of integrated circuits typically includes one or more steps involving the etching of vias, trenches and the like, especially during back-end-of-line (BEOL) processing. These etching processes generally leave residues at the bottom and on the sidewalls of the etched structures. The residues are often organo-metallic polymers containing species such as C, O, F, Si, Cu, H and N. The presence of these residues contributes to problems such as low yield, high via resistance, via voids and reliability problems. Thus, fabrication processes include a post-etch cleaning step designed to remove these residues.
Conventionally, wet cleaning techniques are used to remove such residual polymers, notably by the application of aqueous solutions, such as dilute hydrofluoric acid (HF), or organic acids/bases.
More recently, complex multi-component solvent systems (including components such as corrosion inhibitors, chelating agents, fluorine, amines and other chemical compounds) have been developed for use as post-via-etch cleaning formulations targeted at the particular processing residues formed in specific fabrication lines. Furthermore, WO2005/008739 proposes use of aqueous micellar solutions for cleaning non-porous low-k dielectric materials, reasoning that the low chemical reactivity of such solutions will leave k values of the dielectric layers unaffected.
Now, low-k and ultra-low-k dielectric materials are becoming employed to an increasing extent in integrated circuit devices. Many of these materials are porous in order to achieve the desired low value of dielectric constant. Indeed, often a known low-k dielectric material has porosity introduced therein in order to reduce its dielectric constant yet further.
Some porous low-k dielectric materials that have recently been proposed include porous materials deposited by chemical vapour deposition (CVD) processes: such as later versions of Black Diamond (namely Black Diamond II and III, porous low-k SiCOH) made by Applied Materials Inc. of California, USA, and Orion made by Trikon Technologies Inc of Newport, UK; and spin-on dielectric materials: such as p-SiLK (the porous version of SiLK) made by Dow Chemicals, Zirkon LK (porous methyl silsesquioxane) made by Rohm and Haas' subsidiary Shipley, LKD-5109 made by JSR Corp of Japan and Aurora 2.7 and Aurora ULK (carbon-doped silicon oxide) made by ASM International NV of Bilthoven, Netherlands.
However, when an etched structure is formed in a porous material, many conventional post-via-etch wet cleaning approaches have been found to give undesirable results. More particularly, when the conventional approach is used for post-via-etch wet cleaning of, for example, a wafer bearing a porous interconnect layer, the porous dielectric material has been found to adsorb chemicals and liquids (such as water) from the cleaning fluids, leading to an undesired increase in the dielectric constant. The reason for this problem is explained below.
Any silicon oxide-containing material will have a substantial population of surface hydroxyl (silanol) groups on the surface, which are highly polarized and therefore have a high affinity to water uptake. These sites are generated by the break up of four and six member bulk siloxane (Si—O—Si) bridges at the surface of the material. These siloxane structures at the material surface have an uncompensated electric potential and so can be considered to be “strained”. They will react with moisture to form surface hydroxyl groups. When the material is porous, the surface hydroxyls and the adsorbed water molecule will propagate to the bulk of the material. This may result in increased dielectric constant values and reduced reliability of the film. Similar adsorption mechanisms operate for the chemicals present in wet cleaning fluids. In addition, the chemicals can remain strongly adsorbed within the porous structure, resulting in an increase in K value.
A comparable effect occurs for other materials, such as metal oxides, present on the surface of a wafer. The metal ion-oxide bonds located at the surface of the material have an uncompensated electric potential. This leads to a tendency to react with moisture so as to form surface hydroxyl groups. Once again, if the material is porous the surface hydroxyls and adsorbed water molecule will propagate to the bulk of the material and lead to an unwanted increase in dielectric constant. Once again, similar adsorption mechanisms lead to the take-up of chemicals from the wet cleaning fluid, which can be more severe than water intake as chemicals cannot be removed by drying whereas water can (at least to some extent).
For example, tests have been conducted using a sample made of the above-mentioned porous ULK material LKD-5109, having dielectric constant k 2.25. When structures are etched in this sample and cleaned with known cleaning liquids then it has been found that the dielectric constant of the sample increases disadvantageously, primarily because chemicals penetrate the pores and cause an increase in K value. For example, using aqueous-based cleaning liquid Deerclean (or LK1) from Kanto Corporation it was found that the dielectric constant increases to 2.4; using solvent-based cleaning liquid ESC 760 manufactured by ATMI, dielectric constant increases to 2.6; using solvent-based cleaning liquid ST-250, manufactured by ATMI, dielectric constant increases to 2.9; using solvent-based cleaning liquid CLk 870 or CLk 888, manufactured by Malinckrodt Baker Inc., dielectric constant increases to 2.8.
The problem of take-up of chemicals into the porous dielectric layer is particularly acute in the case where the porous dielectric has undergone plasma damage during the via/trench-etching process. In addition to the negative effect on the porous dielectric layer's dielectric constant, the adsorbed chemical and moisture can also cause problems during subsequent stages in the manufacture of the circuit, notably degassing and reliability problems.
In view of the above-mentioned problems, some proposals have been made to counteract the liquid uptake by the porous dielectric material that occurs during post-etch wet cleaning.
According to one such proposal, after the etching step a separate process is applied in order to seal the exposed pores in the porous material before application of post-etch cleaning liquids. However it has been found that, after cleaning such pore-sealed dielectric layers using conventional cleaning fluids there has still been undesirable water adsorption by the dielectric layer. Moreover, such an approach has the disadvantage of adding a step to the fabrication process, increasing fabrication time and costs.
Another recent proposal of this type, in EP-A-1,511,072, involves annealing the porous layer after the post-etch cleaning step, so as to drive off the liquid components that have penetrated into the porous material. However, this proposal also has the disadvantage of increasing the number of steps involved in the fabrication process.
In view of the above results, up to now it has been believed that wet cleaning techniques are unsuitable for use in cleaning structures etched in porous materials or, at the least, they need to be combined with an additional process to counteract the ingress of liquid into the porous material.
An alternative “dry” cleaning technique has been proposed for post-via-etch cleaning of a wafer bearing a porous dielectric material. This approach involves applying supercritical carbon dioxide (CO2) to the etched surface. However, this approach has the disadvantage that it requires investment in new equipment which is at a more experimental stage in development than the cleaning equipment already in widespread use in the semiconductor manufacturing industry.
There is a need for an improved technique for treatment of structures etched in porous low-k materials.